Identification cards such as credit cards, bank debit cards and the like are rapidly replacing cash as a preferred method for conducting commercial transactions. Consequently, forgery and fraudulent use of such cards is an increasing problem. Cards which carry photographs provide a fast and convenient means of identifying the authorized user. Fraudulent use of photographic identification cards typically involves substitution of another photograph for that of the authorized user. Cards may be made resistant to such tampering either by increasing the difficulty of photograph substitution, or by providing independent means for verifying the authenticity of the photograph on the card. Protecting the photograph against substitution significantly increases the production cost of the card, while independent verification means are generally either inconvenient to access or susceptible to forgery themselves.
U.S. Pat. No. 4,596,409, describes an identification card having a tamper-resistant grey-scale image of the authorized card user burnt into the surface of the card. The opaque plastic card body is dyed with a white titanium dioxide pigment and the image is recorded with a laser beam. The laser beam energy is controlled in such a way as to produce local irreversibly discolored zones which extend from the surface into an inner area of the card body. Discoloration is produced without destruction of the card material either by direct control of the beam strength, or by varying the length of an "impulse package" of beam energy. This method of laser writing is not practical for recording high density digital information.
U.S. Pat. No. 4,680,459 discloses an updatable micrographic data card with a strip of reflective direct-read-after-write (DRAW) laser recording material. Microscopically visible characters composed of groups of smaller laser-written spots are recorded in situ onto the strip. A magnifier with at least 10.times.magnification is required to read the characters. Character representation formats are not suitable for representing detailed images, such as photographs, which contain continuous shades of grey.
It is therefore an object of the present invention to provide an improved identification card having a tamper-resistant macroscopic image of the authorized card user for fast convenient verification of the authenticity of photographic or other identification data.
It is a further object to provide an improved identification card having a macroscopic verification image in a laser recording material suitable for digital recording of high-density optical data. SUMMARY OF THE INVENTION
The above objects have been met with a method for recording a macroscopic image of the authorized user on the reflective laser recording material on an optical data storage card.
A dither matrix of pixel threshold values is applied to a high resolution image of the card user's face, such as a photograph or digital image file, to produce a bi-level representation of the image. A laser beam records each pixel which exceeds its associated threshold value as an array of microscopic laser spots or laser lines. The laser recording process produces a change in reflectivity of the recording material either by ablation, melting, physical or chemical change.
The uniform surface reflectivity of the laser recording material before recording typically ranges between 8% and 65%. For highly reflective material the average reflectivity over a laser-recorded spot might be in the range of 5% to 25%. Thus, the reflective contrast ratio of the recorded spots would range between 2:1 and 7:1. Laser recording materials are known in the art that create either low reflectivity spots in a moderate to high reflectivity field or high reflectivity spots in a low reflectivity field. An example of the latter type is described in U.S. Pat. No. 4,343,879. When the reflectivity of the field is in the range of 8% to 20% the reflective spots have a reflectivity of about 40%. The reflective contrast ratio would range from 2:1 to 5:1. No processing after laser recording is required when DRAW material is employed. Laser recording materials that require heat processing after laser recording may also be used.
The threshold value for individual pixels in the image is computed using a dither matrix, such as a four-by-four matrix of threshold values, which is applied to four-by-four pixel blocks in a repeated manner to cover the entire image. Each threshold-exceeding pixel in the image is represented by an array of microscopic laser spots or microscopic laser lines. This allows a macroscopic image to be recorded on the card by the same laser recording apparatus used for digitally recording high-density optical data. A tamper-resistant verification image is thus provided without increasing the production cost of the card.